1. Field of the Invention
The present invention relates to a disk drive device adapted for use with a disk-shaped recording medium in order to perform a recording and a reproducing operation thereon, and more particularly to a technology for performing a seek operation to a target location on a disk-shaped recording medium having a layered structure with a plurality of signal-record surfaces.
2. Description of the Related Art
For example, compact disks, such as what are called compact disk-read only memories (CD-ROMs), and digital versatile disks/digital video disks (DVDS) have been developed as optical-disk-type recording media.
In disk drive devices adapted for use with these types of optical disk, tracks on the disk being rotated by a spindle motor are irradiated with laser beams from an optical pickup. Data on the disk is read by detecting laser beams reflected from the disk. Data is recorded onto the disk by irradiating laser beams modulated by recording data.
In order to perform a recording and a reproducing operation using laser beams, a laser beam spot needs to be kept focused on a recording surface of the disk. For this reason, a focus servo mechanism is incorporated in the disk drive device in order to control the state of focus of the laser beam spot by moving an objective lens, being a laser beam output end, towards or away from the disk. The focus servo mechanism usually comprises a biaxial mechanism and a focus servo circuit system. The biaxial mechanism comprises a focusing coil and a tracking coil. The focusing coil is used to move the objective lens so that it moves towards and away from the disk. The tracking coil is used to move the objective lens in a radial direction of the disk. The focus servo circuit system is used to generate a focus error signal (or a signal indicating the amount of displacement from the focused-state location) based on information carried by the laser beams reflected from the disk. Then, based on the generated focus error signal, it generates a focus drive signal, which is applied to the focusing coil of the biaxial mechanism.
In other words, the focus servo mechanism is constructed as a feedback control system.
It is well known that the range within which the objective lens can be drawn in to bring it to a focused state based on the focus error signal is a very narrow range where an S-shaped curve of the focus error signal is observed. Therefore, in order to properly operate a focus servo, it is, in general, necessary to carry out a focus search operation as an operation to be carried out when a focus servo loop is turned on.
In the focus search operation, a focus drive signal is applied to the focusing coil to force the objective lens to move in a focusing stroke range. At this time, when a focus error signal is observed, an S-shaped curve is observed when the objective lens is located in a certain range. The focus servo is turned on at a timing in which a linear area of the S-shaped curve of the focus error signal appears (or at a zero cross timing).
Some types of disk have a layered structure with a plurality of recording surfaces. For example, the above-described DVD generally has two signal-record surfaces, called layer 0 and layer 1.
The structure of a DVD with two signal-record surfaces is illustrated in FIG. 7.
As illustrated in FIG. 7, the diameter and thickness of the DVD are 12 cm and 1.2 mm, respectively.
The DVD layered structure includes a disk substrate (or a transparent layer) 101, which is formed at a disk surface 108 side. The disk substrate 101 is formed of transparent synthetic resin with high light transmittance and mechanical and chemical resistant properties. Examples of synthetic resin include transparent polycarbonate resin, polyvinyl chloride resin, and acrylic resin.
Pits are transferred onto one of the main surfaces of the disk substrate 101 by a stamper incorporated in a die in order to form a first signal-record surface 102. The pits in the first signal-record surface 102 are small holes in the disk substrate 101 and form recording tracks. The small holes are encoded holes with different circumferential lengths in correspondence with predetermined information signals.
Through a first reflective layer 103 formed in correspondence with the first signal-record surface 102, a second signal-record surface 104 and a second reflective layer 105 formed in correspondence with the second signal-record surface 104 are formed. Similarly to the first signal-record surface 102, pits are also formed in the second signal-record surface 104 in correspondence with information signals.
An adhesive surface 106 is formed on the second reflective layer 105. A dummy plate 107 is adhered to the second reflective layer 105 through the adhesive surface 106.
Laser beams from the disk drive device strike the DVD from the disk-surface-108 side. Data recorded on the first signal-record surface 102 or the second signal-record surface 104 is detected from laser beams reflected therefrom.
The first reflective layer 103 is a semi-transparent film which allows a certain proportion of the laser beams to be reflected. Therefore, when the laser beams are focused on the first signal-record surface 102, a signal recorded on the first signal-record surface 102 can be read from the laser beams reflected from the first reflective layer 103. On the other hand, when the laser beams are to be focused on the second signal-record surface 104, the laser beams pass through the first reflective layer 103 and are gathered at the second signal-record surface 104, allowing a signal recorded on the second signal-record surface 104 to be read from the laser beams reflected from the second reflective layer 105.
When a disk having a plurality of signal-record surfaces, such as a two-layered DVD, is used, the focus servo mechanism needs to focus laser beams on each of the signal-record surfaces. In other words, the focus servo mechanism needs to function in such a way that when laser beams are focused on a signal-record surface, a focus jump operation is executed to focus the light beams on another signal-record surface.
The focus jump operation is executed by turning off the focus servo and forcing the objective lens to move when the laser beams are focused on a signal-record surface, and turning on the focus servo at a moment the objective lens arrives within a focusing drawing-in range with respect to another signal-record surface (that is, at a moment an S-shaped curve is observed). In other words, the focus jump operation is carried out similarly to the above-described focus search operation.
The focus search operation may, for example, be carried out when making an access to a required address at another signal-record surface from the signal-record surface currently being subjected to a recording or a reproducing operation when the objective lens is in focus.
Here, one general way of carrying out the seek operation is to execute focus jumping from a current address location at a signal-record surface to another signal-record surface in order to seek a target location at the signal-record surface reached by the focus jump operation.
In general, when a focus jump operation is to be carried out, considering shaking of the disk surface, the objective lens is moved with an acceleration which is larger than the disk-surface-shaking acceleration in order to reduce the effects of outside disturbances.
However, when a disk which shakes greatly is used, the focus of the objective lens is considerably displaced within a rotational period of the disk. In addition, since the disk rotates while its center is being supported, disk-surface shaking becomes greater towards the outer periphery of the disk. Therefore, even when a focus jump operation is carried out by performing an objective lens movement control operation which takes into account disk-surface shaking to a certain extent, the focus jump operation may not be successfully carried out because a compromise needs to be made between the amount of disk-surface shaking and the focus jump location. In this case, a re-try operation is executed until the focus jump operation is completed. This results in a delay in starting a reproducing operation at a target location.